The heart is a large muscular organ with the very important job of circulating blood through the blood vessels to the body. Located in the center of the chest, the heart is the hardest working muscle in the human body — always working, even while we are sleeping. The heart and blood vessels together make up the body's cardiovascular system and are vital to supplying the body with the necessary oxygen and nutrients needed to survive. When you breathe, your lungs take in oxygen. The heart pumps blood to the lungs to pick up oxygen, and then it pumps blood through the body to deliver that oxygen.
The animations below show how a normal heart pumps blood. They also explain the changes that happen to a normal heart right after the fetus is born.
The structure of the heart
The heart is a two-sided pump made up of four chambers: the upper two chambers called atria and the lower two called the ventricles.
- On the right side of the heart, the right atrium and right ventricle work to pump oxygen-poor blood returning from the body back to the lungs to be reoxygenated.
- On the left side of the heart, the left atrium and left ventricle combine to pump oxygenated blood back through the body.
- Muscular walls, called septa or septum, divide the heart into two sides and keep the two kinds of blood from mixing.
There are four valves within the heart. Each valve has flaps that prevent blood from flowing in the wrong direction — opening to allow forward flow of blood and closing to prevent backward flow.
- The tricuspid valve is between the right atrium and the right ventricle
- The pulmonary valve is between the right ventricle and the pulmonary artery
- The mitral valve is between the left atrium and the left ventricle
- The aortic valve is located between the left ventricle and the aorta
How blood circulates through the heart and body
The four chambers of the heart are attached to major veins or arteries that either bring blood into or carry blood away from the heart.
- The two largest veins in the body, the superior and inferior vena cava, bring the oxygen-poor (blue) blood to the heart into the right atrium.
- This oxygen-poor blood then passes through the tricuspid valve into the right ventricle. The tricuspid valve closes after the blood passes through to prevent it from flowing back into the right atrium.
- The right ventricle fills and contracts to pump blood to the lungs.
- The right ventricle pumps blood through the pulmonary valve into the pulmonary artery. Once blood passes into the pulmonary artery the pulmonary valve closes to prevent backflow of blood into the right ventricle.
- The two branches of the pulmonary artery carry blood to both lungs.
- In the lungs, the blood picks up oxygen and expels carbon dioxide.
- Reoxygenated blood (red blood) leaving the lungs enters the heart through the pulmonary veins and is carried into the left atrium.
- The reoxygenated blood then flows through the mitral valve and into the left ventricle. The mitral valve closes after the blood passes through to prevent backflow.
- The left ventricle, the most muscular chamber of the heart, then contracts with enough pressure to send the blood through the aortic valve and into the aorta. After the blood passes through the aortic valve it closes to prevent backflow of blood into the left ventricle.
- The aorta branches into arteries, arterioles and capillaries as it delivers blood throughout the body.
- At the capillaries the blood delivers its supply of oxygen and picks up carbon dioxide. It then begins its journey back to the heart, through the veins, and back into the inferior and superior vena cava for the process to begin again.
The heart's control system
A heartbeat is caused by an electrical impulse traveling through the heart. The heart's built-in electrical system controls the speed of its pumping. The electrical impulse originates in the sinus node which functions as the heart's natural pacemaker. The sinus node is most often located in the top of the right atrium. The electrical signals travel through the heart tissue causing the atria and ventricles to contract and relax and the blood to be pumped to the body.